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A. S. Ramachandra Murty
and
Bh V. Ramana Murty

Abstract

In an attempt to investigate conditions under which ice crystals can form at comparatively warm temperatures in supercooled regions of a cloud, the scope of experiments on drop freezing, previously undertaken by the authors, has been enlarged. The results of the present extensive series of experiments have confirmed that supercooling drops, when subjected to evaporation, freeze more readily due to what has been called the “dynamic effect” of evaporation. When sodium sulphate was added to the drops in arbitrary concentration, their freezing probability also showed a significant inctease. These findings indicate that, in supercooled regions of a cloud where prevailing conditions are similar to those of the reported experiments, the ice crystal concentration should be two to three orders of magnitude higher than the existing ice nucleus concentration.

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L. T. Khemani
and
Bh V. Ramana Murty

Abstract

The rainfall data, for the period 1901–69, of three stations in the region downwind of the urban industrial complex at Bombay and of two stations in the nearby non-urban region, have been analyzed. The study has indicated that, with respect to the non-urban region, the region downwind of the urban industrial complex recorded an increase of rainfall by about 15%, significant at less than the 1% level, during 1941–69 which is the period of increased industrialization.

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B. K. MUKHERJEE
and
Bh. V. RAMANA MURTY

Abstract

Examination of the available rocketsonde data for the tropical station, Thumba, India, has shown that warmings, less pronounced than in the case of middle and high-latitude stations but of noticeable magnitude, and coolings of similar magnitude occurred in the mesopheric and upper stratospheric levels during the period, December 1970–March 1971. This was the only winter period when observations were made at the station during all of the 4 mo. No prominent change in wind has been observed in association with the warnings, however.

The maximum warming observed over a period of 1 week in the upper stratosphere was 26°C at 45 km. The upper mesosphere had been subjected to a continuous process of warming for over 3 weeks in December–January 1971 during which period the temperature rose by 48°C at 70 km. There is no definitive indication that these warmings were of the propagating type. However, they appear to have moved in the vertical at a rate of 3–5 km/day.

The temperature behavior of the lower stratosphere (50 mb) and the upper troposphere (300 mb) in the winter of 1970–71 was different than that which was observed in the preceding 2 winters.

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Bh V. Ramana Murty
,
A. K. Roy
, and
K. R. Biswas

Abstract

The study of echo intensity profile below bright band, when this appears on radar in certain rain situations, has shown that raindrops, after leaving melting level and falling through clouds in warmer layers below do not, as a rule, undergo any noticeable increase in their sizes, suggesting that lower level clouds at this stage have little or no significant liquid water content. The implications of this on the life cycle of rain situations associated with development of bright band are discussed, with reference to a few instances of study, using radar, of such rain at Delhi.

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B. K. Mukherjee
,
R. S. Reddy
, and
Bh V. Ramana Murty

Abstract

Temperature and wind data for the troposphere, stratosphere and mesosphere obtained from rocket-sonde/radiosonde/rawin observations made at a tropical station (Thumba, 8°32′15″N, 76°51′48″E) during five summer monsoons (1971–73, 1975–76) with differential monsoon activity were examined.

There is agreement between the occurrence of high-level warmings and monsoon activity in four out of five monsoons studied. There were no warmings in the year with very weak monsoon activity. The temperatures of the stratopause and the tropopause were significantly warmer in 1972 when the monsoon was very weak than in other years when the monsoon was active or very active.

There is a high positive correlation between the monsoonal activity (precipitation departure from normal over Indian subcontinent) and the 25 km mean zonal wind, and a strong negative correlation with the winds near 16 and 50 km. The change in the sign of correlation coefficient was due to the observed phase change with altitude of the quasi-biennial oscillation.

The study indicated the possibility of a relationship between stratospheric quasi-biennial structure and the Indian monsoon rainfall.

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B. K. Mukherjee
,
K. S. Rao
, and
Bh V. Ramana Murty

Abstract

Computations of vertical motions in the middle atmosphere over the Indian tropical region have been made based on the thermodynamic equation with the geostrophic approximation. The authors have used the once weekly rocketsonde temperature and wind data for the tropical station Thumba India, (8°32′N, 76°52′E) for the four summers (1972, 1973, 1975 and 1976) and two winters (1971 and 1972) which are also years of varying monsoon activity.

In the tropical middle atmosphere, downward motion (subsidence) is the dominant feature when the motion field is considered in a longer time scale. The trend of fluctuations in vertical motion suggests wave structures in the tropical middle atmosphere. The magnitude of the extreme values of the vertical motion in the stratosphere over the high latitudes is larger, by a factor of 2 or more, than those obtained over the low latitudes (tropics). Whereas the magnitude of the vertical motion in high latitudes is associated with stratospheric warmings during winter, the values relating, to low latitudes are computed at 3 h intervals during March.

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A. Mary Selvam
,
G. K. Manohar
,
L. T. Khemani
, and
Bh V. Ramana Murty

Abstract

Simultaneous measurements of raindrop charge, atmospheric electric field and rain intensity were made at Poona using fast response, continuous recording surface instruments during four types of rain: pre-monsoon (thunderstorm rain), monsoon rain type I (tight intermittent rain), monsoon rain type II (heavy continuous rain) and post-monsoon (thunderstorm rain). Measurements were also made of the conductivity of rainwater samples collected during the monsoon season. On some days of this period data on cloud thickness as obtained from the aircraft flights in the region were also available.

The electric field associated with negatively charged raindrops was less negative than that associated with positively charged drops. The raindrop charge spectrum showed different characteristics during the four types of rain studied. It was broadest during the pre-monsoon rain when the convective activity was a maximum. It followed a log-normal distribution during the monsoon rain type I and it was peaked and skewed to the positive side during the monsoon rain type II. Rainwater conductivity was inversely correlated with the cloud vertical thickness. The results of the present study support the warm cloud charge generation mechanism proposed by Takahashi (1974).

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K. R. Biswas
,
R. K. Kapoor
,
K. K. Kanuga
, and
Bh V. Ramana Murty

Abstract

An experiment on artificial stimulation of rain using a warm cloud seeding technique was undertaken in three nearby climatologically similar regions, Delhi, Agra and Jaipur in northwest India. Analysis of the data from 18 experiment-seasons has suggested a positive trend of the result, which is found significant by statistical tests.

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A. S. Ramachandra Murty
,
A. M. Selvam
,
R. Vijayakumar
,
S. K. Paul
, and
Bh V. Ramana Murty

Abstract

Electrical and microphysical measurements were made in warm maritime and inland cumulus clouds, before and after seeding, by repeated aircraft penetrations at a single level, a few hundred meters above cloud base. Such measurements were also made in non-seeded clouds by single penetrations during transient flights.

The electric field initially was negative in the maritime clouds which developed rain. In the cloud case which dissipated without rain it was initially positive. The field showed sign reversal with time, occasionally preceded by intensification, in all maritime clouds. The field initially was positive in inland clouds. It showed no time variation except in one cloud case where both positive and negative fields were recorded during the period of heavy rain.

The droplet charge, droplet median volume diameter and liquid water content showed no marked time variation in either maritime or inland clouds. However, in the cloud case which developed heavy rain marked increases in droplet median volume diameter and liquid water content were recorded.

The time variations of electrical and microphysical parameters following seeding are in general within the range of their natural variability.

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